Mass scanning mass spectrometers, such as quadrupole mass spectrometers, are ubiquitous analytical devices. A major drawback of any scanning mass spectrometer is a loss of sensitivity due to poor duty cycle. For example, if a quadrupole mass spectrometer scans a mass range of x Da with a mass resolution or peak width of y Da, then a duty cycle of y/x is obtained. For a conventional quadrupole mass spectrometer, realistic values of x and y are 1000 and 1 respectively, resulting in a duty cycle of only 1/1000 or 0.1%. Physically, this is because when scanning in this way the spectrometer is only detecting 0.1% of the total mass range at any instance in time; all of the other ions are unstable and so are rejected.
The charge on an ion, q, can be rewritten as ze where e is the electronic charge and z is the so called charge state of an ion. It has been previously recognised that ions of differing charge state may occupy differing radial positions within a multipole ion guide [Rapid Communication in Mass Spectrometry 14, 1907-1913 (2000)]. In this study it was shown how ions of similar m and differing z or similar z and differing m occupy differing radial positions in a multipole ion guide when in the presence of a buffer gas. This stratification is caused by both the space charge repulsion between differing species and the charge and mass dependence of the effective potential. The behaviour of such devices depends on the ion density present within the guide and as such it is difficult to exploit this behaviour as a predictable deterministic analytical separation. This is because at any one time the number and type of ion species within the guide and therefore its space charge can vary dramatically. It would be desirable to produce a device that separates ions in a predictable manner enabling it to be efficiently coupled to further spectrometer stages downstream from the device.
The use of radio frequency (RF) ion guides at elevated pressures to efficiently transmit ions from one portion of a spectrometer to another is now widespread. These devices work on the principle of so called “effective potential wells” (Gerlich et al, (1992) Inhomogeneous Electrical Radio Frequency Fields: A versatile tool for the study of processes with slow ions. Adv. In Chem Phys LXXXII, 1.ISBN 0-471-53258-4, John Wiley and Sons). Ions may be trapped in these wells for extended periods of time either by the use of cylindrical geometry devices such as conventional Paul traps, or using linear geometry devices such as multipole guides or ring sets with end plates providing trapping D.C. potential. These RF devices are able to trap in three dimensions in a way which is impossible to achieve using purely electrostatic ion optical elements. This is because Laplace's equation, which describes the behaviour of electrostatic fields, contains no true potential minima but only saddle points which on their own are insufficient to give true three dimensional trapping. An oscillatory A.C. field applied to quadrupoles, hexapoles, and octopoles (collectively known as multipoles) or to ring sets gives rise to the so called ponderomotive force which acts in the direction of weaker field i.e. towards the central optic axis of the multipole or ring set. In the absence of gas, ions will oscillate in the potential well with an amplitude dependent upon their radial energy. In the relatively simple case of quadrupoles the restoring force towards the optic axis is proportional to the distance from it and so an ion with finite energy may be seen to exhibit simple harmonic motion on a macroscopic level within the well. The addition of gas molecules to such a device acts to dampen this radial motion so that ions are in effect cooled and concentrated to the centre of the device. As ions are cooled in these linear multipoles or ring sets they loose any forward impetus they had to traverse the length of the device. The ions are rapidly thermalised and will remain in the guide until the space charge effect from other ions behind pushes them along. This sluggish motion of ions in the guides has led to problems when interfacing with fast scanning devices such as analytical quadrupoles. U.S. Pat. No. 4,283,626 describes the use of a “leaky dielectric” inserted inside the multipole to allow for the provision of a drift field to speed the ions through a collision cell. This leaky dielectric is transparent to the RF field (thus maintaining a potential well) but has enough resistivity to allow a potential gradient to be applied axially along its length. U.S. Pat. No. 4,283,626 recognises that such a drift field in the presence of gas may be used to separate ions for analytical purposes. U.S. Pat. No. 5,847,386 describes a number of different methods to induce a smooth axial field along the length of the linear guides to speed the transmission of ions through them. Such methods include segmenting of the rods themselves, or using external ring electrodes, or tapering the rods themselves or using different pitch circle diameters for oppositely phased rod sets at either end of the guide. US patent publication 2002/0070338 describes the use of segmented rods to provide an axial D.C. field and to give separation of the ion species according to their ion mobility. Again, RF confinement is combined with a drift field in the presence of gas. This combination is versatile since ions may be manipulated in a wide variety of ways using D.C travelling waves in the axial direction to create moving potential wells while maintaining radial confinement with the ponderomotive force from the RF supply. Related techniques are described in U.S. Pat. No. 5,206,506 and U.S. Pat. No. 6,483,109. The contents of U.S. Pat. No. 4,283,626; U.S. Pat. No. 5,847,386; US 2002/0070338; U.S. Pat. No. 5,206,506 and U.S. Pat. No. 6,483,109, together with U.S. Pat. Nos. 6,794,640 and 6,800,846, are hereby incorporated by reference. With the exception of Gerlich, all of the above techniques describe devices using RF ponderomotive confinement in both dimensions, i.e. they confine ions radially simultaneously but provide little or no radial spatial separation of ions. Gerlich describes a stacked rf plate ion guide with DC top and bottom plates which is employed as a storage ion source, but no theoretical treatment of this device is presented.